Process for making conductive polymers

ABSTRACT

Conductive polymer having a steeply-sloped positive temperature coefficient (PTC) of resistance with a sharply defined anomaly temperature is provided by selecting a polymer having crystalline structure at room temperature, a glass transition of approximately 13°F. or lower and a narrow molecular weight distribution as measured by a flow ratio of less than 9 or a melting point range of 30°C. or less. The polymer is filled with conductive particles and may include additional additives such as stabilizing agents and flame retardants. The ingredients are mixed, subjected to shearing forces and then formed into the desired configuration. The maximum permissible processing temperature is identified for the specific polymers employed.

This is a division of application Ser. No. 210,648, filed Dec. 22, 1971,now U.S. Pat. No. 3,760,495, which is in turn a continuation - in - partof application Ser. No. 6,086, filed Jan. 27, 1970, which issued on June27, 1972 as U.S. Pat. No. 3,673,121.

This invention relates to electrically conductive polymer, and moreparticularly to polymeric material having a controlled steeply slopedpositive temperature coefficient (PTC) of resistance.

An object of the invention is the provision of conductive polymericmaterial having an improved PTC characteristic. Another object is theprovision of a method for making such material. Yet another object isthe provision of PTC polymeric materials and process for making samewhich obviates the disadvantages of the prior art.

In the accompanying drawings, in which several of the various possibleembodiments of the invention are illustrated;

FIG. 1 is a resistivity versus temperature curve of an electricallyconductive polymeric element made in accordance with this invention;

FIG. 2 illustrates the main process steps employed in making such anelectrically conductive polymeric element;

FIG. 3 is a cross section of a mold useful in forming an element fromthe polymeric mixture of this invention;

FIG. 4 is a pictorial view of polymeric form as it is removed from theFIG. 3 mold;

FIG. 5 is a pictorial view of the FIG. 4 form after it has been machinedinto an elongated annular element;

FIG. 6 is a variation of the FIG. 5 polymeric element; and

FIG. 7 is a DSC plot for a typical polymer.

Dimensions of certain of the parts as shown in the drawings may havebeen modified or exaggerated for the purpose of clarity of illustration.

The invention accordingly comprises the elements and combination ofelements, compositions, methods, features of construction, andarrangements of parts which will be exemplified in the structures, stepsand sequence of steps hereinafter described, and the scope of theapplication of which will be indicated in the following claims.

Conductive polymers having a PTC effect are known in the art. See forinstance U.S. Pat. Nos. 2,918,665 and 3,243,753. Such polymers areuseful for instance in electrical circuits as a sensor sensing ambienttemperature or as a heater with an inherent function of current limitingthereby obviating the need for thermostats or other current limitingdevices. Further, polymeric materials offer the advantage of permittingrelatively low cost fabrication techniques such as molding and extrudingwhile being readily machinable. Used as a heater an element constructedout of such polymer is connected to line voltage so that current flowstherethrough, causing I² R heating. When this temperature rises abovethe anomaly point, there is a sudden and marked increase in resistanceto effectively cut off current through the heater with heat dissipationmatching heat generation. Making such polymers has been a problemhowever since if the teaching of the prior art, such as the patentsreferred to supra, is followed inconsistent results are obtained. Thatis, although conductive polymers having a PTC effect are known, theprior art does not teach how to make conductive polymers having thedesired characteristics of a controlled steeply-sloped PTCcharacteristic with an anomaly at a chosen temperature and with a basal(room temperature) resistivity at a desired level. Further, due to thelack of understanding of the mechanism giving rise to the PTCcharacteristic the prior art processing techniques have not beensatisfactory. The prior art has generally taught that a PTCcharacteristic in conductive polymers has been caused by a difference inthermal expansion between the polymer material and the conductivefiller, that is, it was believed that if the polymer had a greaterthermal coefficient of expansion than the conductive filler particles,this would give rise to a PTC effect. As the temperature is raised, thepolymer expands more than the conductive particles, thus spreading theconductive particles apart. However, this is not a satisfactoryexplanation since many materials, such as polyvinylchloride andpolystyrene do not exhibit a marked PTC effect even though the thermalcoefficient of expansion of the polymer is greater than the conductiveparticle.

Rather than being primarily dependent upon rates of thermal expansion,the PTC effect appears related to the phase change in polymers havingcrystalline structure and a narrow molecular weight distribution. When acrystalline type polymer, such as polyethylene, is loaded with carbonparticles, such as carbon black, the carbon black is distributedunevenly in the polymer even with extensive mixing. So calledcrystalline polymers include amorphous regions, normally to an extent ofup to 30% by volume and into which the carbon particles movepreferentially when the molten polymer is cooled after mixing. With theproper carbon loading and thorough dispersion, the carbon particles formlarge aggregates separated by crystalline regions with the separationbeing in the order of several hundred angstroms. The polymer willcontain a few chains of carbon particles forming a continuous chainthrough the material but the bulk of such chains will be broken up bycrystalline regions of polyethylene. Electron tunneling can occur fairlyreadily through thin films of crystalline polymer so that carbon chainsbroken up by crystalline regions can have conductivities approachingthose of carbon chains. As temperature rises, the carbon black massesseparate due to the greater thermal expansion coefficient of the polymercompared to the carbon particles, increasing the difficulty of electrontunneling between carbon masses which offsets the increased electrontunneling effect due to temperature rise while the crystalline regionsremain intact. This may be seen in FIG. 1 which is a typical temperatureversus resistivity curve for a carbon loaded polyethylene polymer madein accordance with the invention. At temperatures below roughly 90°C.,the resistance level is relatively flat or even decreases slightly. Whenthe temperature rises further the carbon black masses move further apartand, more importantly, the macrostructure of the crystalline regions andat higher temperatures the microstructure, is destroyed with aconcomitant reduction in the ability to allow electron tunneling. Theseeffects, especially during the early stages of the crystalline phasechange, give a resultant increase in resistivity as seen in the roughly90°-130°C. range of FIG. 1. Further increase of temperature causes thecrystalline regions to melt completely and the polymer to becomesemi-molten which permits the strained carbon masses to expand in thepolymer and form a partial network of carbon through the materialresulting in an increase in conductivity, as seen in FIG. 1 attemperatures above roughly 130°C.

It has been found that several factors affect the PTC characteristics ofthe material. The material must have crystalline structure at roomtemperature and a glass transition temperature of approximately 13°F. orless. Generally, the greater the crystallinity of the material and thelower the glass transition temperature the more the resistance riseswith increased temperature in the anomaly range, or put in another way,the greater the resistivity ratio. The resistivity ratio is defined asthe maximum resistivity divided by resistivity below the anomalytemperature. It is also found that the narrower the molecular weightdistribution the sharper the knee of the PTC curve will be at thisanomaly temperature.

Molecular weight distribution of polymers however is too difficult tomeasure for practical use. More practical indirect measures can be used,for example in olefines and similar polymers the flow ratio can be usedsince it is a function of molecular weight distribution. The flow ratiois the ratio of the flow rate in a standard melt index with a 10 Kgloading to the Melt Index (ASTM test D-1238). A ratio of less than 9indicates a narrow molecular weight distribution, 9-11 indicatesintermediate distribution while greater than 11 indicates a widedistribution. For certain other polymers such as nylons or rubber types,e.g. Trans 4 polybutadiene, this test is not suitable due to the highmelting temperatures involved; however, these can be measured by thespread or range of the melting temperature. Material having a narrowmolecular weight distribution has a melting point range within 30°C. fora polymer where the heating rate is of the order of 10°C./min. A clearpeak occurs at the melting point T_(M). The melting point range may bedefined as the difference between the temperatures T_(B) and T_(A) whichcorrespond with the intersection of lines, drawn tangentially on thecurve at the point at which maximum slope is reached on either side ofT_(M), with the base line.

Various conductive particles can be used in the practice of theinvention, such as any type of carbon particle and electricallyconductive powder of materials which are not subject to oxidation attemperatures to which the material is subjected during processing.Examples of such powder include tin, silver and gold. The desiredanomaly temperature can be obtained by choosing from a variety ofpolymers.

By controlling the above noted variables, a polymer having a muchimproved PTC effect compared to prior art teaching is achieved as wellas enabling better control of the PTC effect as to steepness of the PTCcurve above the anomaly temperature, sharpness of the knee of the PTCcurve and desired anomaly temperature.

In practicing the invention, a crystalline polymer, such as apolyolefin, a conductive filler, such as a carbon black, and otheradditives are mixed together for several minutes in a conventionalmanner as in a standard V-type blender, then the resulting mixture isplaced in a different mixer to effect a more complete dispersion of thefillers throughout the polymer. A standard Banbury type mixer, has beenfound suitable for this purpose. Essentially, it consists of twointerdigitating screw-like vanes contained in a close fitting housingwhich also mounts a ram used to exert pressure on the mix during mixingif so desired. Heat transfer fluid can be circulated through passages inthe vanes to provide heating or cooling if desired.

In order to avoid deleteriously effecting the resistance characteristicsof the material, it is necessary to prevent the temperature of the mixfrom exceeding the temperature at which oxygen pick up increases for theparticular polymer and carbon loading. In the case of polyolefin loadedwith up to 30% by weight carbon, this maximum temperature is 350°F. Thatis, at temperatures above 350°F., the basal resistivity of the materialbegins to rise. It is also preferred to limit the mixing time in theBanbury to 5 minutes once the temperature reaches 200°F. or again thebasal resistivity increases. This can be offset to some extent byincreasing the carbon black content of the mix; however, as carbon blackloading is increased, the strength of the material decreases as well asultimate life. That is, the material does not have sufficient structuralintegrity to maintain its configuration at such loading levels.

The temperature of the material rises due to internal friction duringthe mixing process. Very little mixing is effected until the temperaturereaches 200°F. Although the reason is not understood, it was found thatinitially mixing for one minute at a reduced speed of approximately 77rpm was important in obtaining good results.

As soon as the mixing step is completed, the material is transferred toa shearing apparatus, such as a two roll mill. Essentially, thiscomprises two rolls whose axes are mounted in parallel relation and witha variable distance between the rolls, with one roll rotating fasterthan the other. The rolls are heated to a temperature between 300° and325°F. The temperature of the rolls is kept lower than in the Banburymixer since there is a greater tendency of the polymer to oxidize due tothe increased exposed surface area associated with the shearing actionof the mill. Best results are obtained if one roll is kept cooler thanthe other, with up to 5°F. differential being acceptable. The mixing ismaintained for approximately 5 minutes, with two material fold-overs perminute. This is accomplished by placing a doctor against the roll,peeling the material from it and folding it over into the nip of therolls, after which the material is ready for forming. It then can beformed by conventional thermoplastic processing into any desiredconfiguration, such as rod or pipe, depending on the desired end use.One possible application is for a heater element for an electricallyheated hair curler as set forth in copending and coassigned applicationSer. No. 6,095 filed Jan. 27, 1970, abandoned in favor ofcontinuation-in-part application Ser. No. 109,414 filed Jan. 25, 1971,which issued on Sept. 5, 1972 as U.S. Pat. No. 3,689,736. For such usean elongated cylindrical annulus 22, as shown in FIG. 5, is useful. Oneway to make element 22 is by using mold 50 shown in FIG. 3. The mixtureis taken hot from the two roll mill, roughly shaped into a cylindricalform and inserted into a mold cavity 56, as seen in FIG. 3. This is donebefore the mixture has had a chance to cool in order to avoid theformation of air bubbles in the mold. The mold is closed and pressureapplied. Mold 50 is formed of platen 52 and bed member 54. Bed member 54is formed with a cylindrically shaped mold cavity 56. An enlargedportion 58 communicates with cavity 56 and forms shelf 60. Passageway 62which extends through bed member 54 permits passage of a heat exchangemedium therethrough. Mold release members 64 are mounted in bed member54 to facilitate movement of platen 52 away from the bed member forremoval of the molded element. Another passageway 66, similar to and forthe same purpose as passageway 62, is provided in platen 52. Cylindricalhub 68 formed on platen 52 closely fits into cavity 58 and rests insurface 60 when fully lowered. Core member 70 extends from hub 68 and isprovided primarily as a heat conducting member to conduct heat into theinner portions of the annular polymeric material. When in the loweredposition in mold cavity 56, as seen in dashed lines, it will be notedthat a slight clearance is provided. The upper portion 72 of core 70 isthreaded to facilitate handling of the polymeric element once molded.The molded element is depicted in FIG. 4 as element 74. Element 74,after removing from the mold, is then machined into the elongatedannulus 22 shown in FIG. 5 by boring it out. Preferably, a roughmachined finish is provided on element 22 both on the inner and outerperipheral surfaces 76, 78 respectively, to facilitate adherence of anelectrically conductive coating placed thereon. In instances where aparticularly high number of heating cycles are required, it is useful toprovide an axial slit 22a as seen in FIG. 6, which mitigates the effectof thermal stress due to repeated expansion and contraction of theelement.

Such an annulus may also be produced by extrusion of a pipe which isthen cut to size. Standard extruding equipment may be used. The polymershould be dry prior to extrusion. Care must be taken to prevent stickingin the sizing die caused by the condensation on the cold die ofstabilizer degradation products released during extrusion. Whenextrusion is completed and the pipe is cut to size the resistance of thecore produced is orders of magnitude greater than desired. It has beenfound that in order to produce a useful core the material must be heatedafter extrusion to 110°C. for at least 36 hours after which time theresistance is at the desired value. Higher temperatures require ashorter heating period but the cores tend to deform. While a lowertemperature of 80°C. is possible for the annealing, the time required isexcessive.

For most uses, several additives to the mixture are found to bebeneficial. For instance, it is found that eventually some degradationof the PTC characteristic occurs in the material. This is referred to asaging. The effect is related to a change in form of the crystallineregions brought on by oxidation. It manifests itself by a continuingincrease in room temperature resistivity and ultimately thedisappearance of the PTC anomaly. Certain stabilizing agents materiallydelay the occurrence of aging in the material. While there are manystabilizers used in polymers, most are not suitable for conductivepolymers. An alkylated polyhydroxy phenol, such as Santovar A, a productof Monsanto Chemical Co., is very effective, especially whenpolyethylene is used as the polymer. Phenylbetanapthylamine, such asAntioxygene MC of Ugine-Kuhlmann, Organic Products Division of F.M.C.s.a., of France is also effective in stabilizing the resistancecharacteristics and reduces sticking problems during extrusion. UnlikeSantovar A, its decomposition products do not condense on the sizingdie. These additions stabilize the basal resistivity but does notmaterially effect the PTC characteristic. However, since thedecomposition products are rather volatile, care must be exercised inprocessing to reduce losses of the additive. Dialkyl phenol-sulfide,such as Santowhite, also a product of Monsanto Chemical Co., is found tobe acceptable for reducing degradation at a metal polymer interface thusreducing contact resistance and is particularly useful if the elementsformed from the mixture are to be plated with copper.

Another characteristic that many polymers have that is undesirable formany applications is that they are not self-extinguishing if heated tothe combustion temperature through overheating. It is found thatantimony oxide is effective in making the material self-extinguishing,that is, once the heat source is removed (whether it be external orinternal through I² R heating), the material will not continue to burn.A highly chlorinated perchloropentacyclodecane, such as Dechlorane plus25 of Hooker Chemical Company is found to be effective. The Dechlorane125 to be effective, however, must be used with antimony oxide.Specifically, it should be noted that although antimony oxide andDechlorane 125 are effective when used in high density polyethylene, theamounts of these fillers to be used are determined by the amount ofconductive filler employed. The total filler used in the mixture shouldnot exceed 50% or the strength will be notably effected. It has beenfound that in general as much flame retardant should be added as isconsistent with maintaining structural integrity of the filled material.An optimum percentage of filler material (carbon black, stabilizers andflame retardants) is 40 %. The amount of conductive filler is determinedby the warm-up time desired.

The range of carbon black useful in making such heaters include 14 to30% by weight of the carbon black plus the polymer with a preferredrange of 14 to 20%. The percentage of carbon black filler is selectedand then the amounts of other additives are calculated from a chosenlevel of fillers.

Polymers useful in producing PTC elements having a high resistivityratio (over approximately 3000 which is required for self control orcurrent limiting) are shown in Table I with their approximate oxygenpick up temperature for the preferred carbon loading of 14-20% and theirglass transition temperature.

                  Table I                                                         ______________________________________                                                          Approximate Glass                                                             oxygen pick up                                                                            transition                                                        temperature temperature                                     Polymer           (°F) (°F)                                     ______________________________________                                        Low Density Polyethylene                                                                        320         -130                                            High Density Polyethylene                                                                       350         -193                                            Polypropylene     420         -4                                              Polyethylene Oxide                                                                              190         -106.6                                          Trans 4 Polybutadiene                                                                           300         -4                                              Polyethyl Acrylate                                                                              350         -11.2                                           ______________________________________                                    

Specific examples illustrative of the invention are given below.

EXAMPLE I

To produce an electric curler heater having a 21/2 minute warm-up time,that is the time necessary for the surface temperature of the centralportion of the curler to reach 62°C. using 120 volt supply, one wasprepared from a batch consisting of 1356 grams: 264 grams of oil furnacecarbon black (Vulcan 3 of Cabot Corporation) having an average particlesize of 30 millimicrons, 108 grams antioxidant (Santovar A), 96 grams ofDechlorane 125 flame retardant, 48 grams of antiomony oxide flameretardant and 840 grams of high density polyethylene (Alathon 7030 of E.I. DuPont deNemours & Co., Inc.) were added in the order recited in a Vblender for several minutes. The resultant blend was added to a Banburymixer at a load speed of 77 rpm for 1 minute in four equal batches, theram lowered at 40 psi and speed raised to 116 rpm for 5 minutes afterthe blend had reached its minimum volume (when the ram bottoms). Coolingwater was circulated through the mixing blades to keep temperature below350°F. except for the last 2 minutes when the cooling water was turnedoff. The temperature remained below 350°F. The material was then placedin a two roll mill with the rolls heated to a temperature of 308 and305°F. respectively. The rolls were initially set at 0.200 inches untilthe material was molten, then the space between the rolls was reduced to0.110 inches. The material was milled for 5 minutes with two materialfoldovers per minute. The material was taken in approximately 140 gramquantities still hot from the two roll mill, roughly shaped by hand andplaced in the mold cavity (as seen in FIG. 4) which was preheated to300°F. The mold was closed and pressure increased gradually to 50 tonsin 11/2 minutes. The pressure was maintained for 5 minutes. Coolingwater was then circulated through the mold while pressure wasmaintained, until temperature decreased below 100°F. The mold was openedand the piece removed. The molded piece was then machined to produce anannulus or tubular cylinder of the following approximate dimensions:outside diameter 17.55 mm., inside diameter 5 mm. and length 55 mm.

EXAMPLE II

Same as Example I except that a hair curler heater having a warm-up timeof one minute with a 120 volt supply was prepared in a batch of 1356grams; 276 grams of oil furnace carbon black, 828 grams of high densitypolyethylene with the remainder of the ingredients being the same.

EXAMPLE III

Same as Example II except that a hair curler heater having a warm-uptime of .5 minutes with a 240 volt supply was prepared.

EXAMPLE IV

50 pounds of a blend containing, on a weight basis, 15% of furnace oilcarbon black having an average particle size of 30 millimicrons (Vulcan3 of Cabot Corporation), 8% of antioxidant (Santovar A of MonsantoInc.), 8% of antimony oxide and 4% of flame retardant (Dechlorane 125 ofHooker Chemical) and the remainder high density polyethylene wasprepared by mixing in a Banbury mixer keeping temperature below 350°F.The blend was then two roll milled and pelletized. The resultantmaterial was extruded on a standard extruder as described above and theresultant pipe cut to size. The cores produced were heated at 110°C. for30 hours. These cores had a resistance of about 50 ohms and were usefulas hair curler heaters having a warm up time of 1 minute.

EXAMPLE V

Same as Example IV except that the mix contained 23% furnace oil carbonblack (Vulcan 3) and 77% polypropylene. The temperature was kept below420°F. and the resultant core had a resistance of 75 ohms.

EXAMPLE VI

Same as Example IV except that the mix contained 30% carbon black(Elftex-5) and 70% trans 4 polybutadiene. The temperature was kept below300°F. and the resultant core had a resistance of 6000 ohms.

In all of the above examples the resistance ratio was observed to beabove 3000.

The heaters were then coated on the inner and outer peripheral surfaceswith an electrically conductive coating, such as electroless nickel andelectroplated tin, as described more fully in copending coassignedapplication Ser. No. 6,093 filed Jan. 27, 1970, now abandoned, orelectroless copper.

After coating with conductive coatings including a layer of electrolessnickel, optimum low contact resistance between the electroless nickelcoating and the polymer material was obtained by annealing the heatersfor at least 2 days and at a temperature up to 110°C. Actually highertemperatures would be useful, however, the heaters begin to physicallydeform at temperatures above 110°C. Electroless copper coating obviatesthe necessity of this annealing. The cores from Example IV were platedwith electroless copper using conventional techniques and did notrequire this annealing stage.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

It is to be understood that the invention is not limited in itsapplication to the details of construction and arrangement of partsillustrated in the accompanying drawings, since the invention is capableof other embodiments and of being practiced or carried out in variousways. Also, it is to be understood that the phraseology or terminologyemployed herein is for the purpose of description and not of limitation.

As may changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense, and it is also intended that the appended claims shall cover allsuch equivalent variations as come within the true spirit and scope ofthe invention.

I claim:
 1. An electrical resistance element having a steeply slopedpositive temperature coefficient of resistance above an anomalytemperature comprising a polymer selected from the group consisting of,polyethylene oxide, trans 4 polybutadiene and polyethyl acrylate, saidpolymer having crystalline structure at a temperature below the anomalytemperature and a glass transition lower than approximately 13° F,conductive particles selected from the group consisting of carbon, tin,silver and gold dispersed throughout the polymer, the conductiveparticles comprising no more than 50% by weight of the polymer andconductive particles, and electrical contacts located on spaced portionsof the element.
 2. An electrical resistance element according to claim 1in which the polymer has a flow ratio of less than 9 whereby the polymerhas a narrow molecular weight distribution so that the transitionbetween the resistance level at a temperature below the anomaly and thesteeply sloped portion of the resistance temperature curve attemperatures above the anomaly is sharpened.
 3. An electrical resistanceelement according to claim 1 in which the polymer has a meltingtemperature range within approximately 30° C. for a heating rate in theorder of 10°C./minute whereby the polymer has a narrow molecular weightdistribution so that the transition between the resistance level at atemperature below the anomaly and the steeply sloped portion of theresistance temperature curve at temperatures above the anomaly issharpened.
 4. An electrical resistance element according to claim 1 inwhich the conductive filler is carbon black from 14 to 20% by weight.